Process for flattening silicon steel sheets



Patented Dec. 3, 1946 PROCESS FOR FLATTENING SILICON STEEL SHEETS CarlE. Giflzord, Zanesville, Victor W. Carpenter, Franklin, and Lowell L.Cook, Zanesville, Ohio, assignors to The American Rolling Mill Company,Middletown, Ohio, a corporation of Ohio No Drawing. Application March28, 1941, Serial No. 385,756

Claims. 1

There has been recently a considerable tightening in the requirementsfor flatness in sheets of electrical steel. It will be understood thatthe flatness characteristics of magnetic sheet metals vary considerablyin accordance with the mode of manufacture of the sheets, so thatdiiferent standards of flatness have been thought to apply todifferently processed materials. While cold reduced strip has beendeveloped to the point where it is commonly quite flat, light gage, hotrolled sheets are at best somewhat wavy and consequently are at acommercial disadvantage for some applications.

It is an object of our invention, therefore, to provide a means wherebythe flatness characteristics of hot rolled sheets may be usefulyimproved. While the degree of improvement will vary somewhat with theflatness of materials before they have been given our treatment, it isnevertheless, following the general object set forth above, an object ofthis invention to provide a treatment which will impart flatnesscharacteristics substantially equal or better than the best flatnesshitherto produced, to this material hitherto characterized by notablelack of flatness, i. e. materials hot rolled to gauge.

It is an object of our invention to provide a flatness treatment whichin itself is not only relatively inexpensive, but which may be practicedupon magnetic materials without impairing the magnetic characteristicspreviously developed therein by other treatments. Further it is anobject of our invention to provide a treatment which is capable, in somemodes of manufacture, of being substituted for a final heat treatmentfor the production of magnetic characteristics. In this way furthereconomies may be effected, as will be set forth hereinafter.

Our invention has been found applicable to all commercial grades of hotreduced electrical steel sheets irrespective of the quantitative siliconcontent, or the content of other alloying ingredients, and theparticular routing of the steel does not form a limitation upon theinvention herein described.

The objects set forth above, and others which will be pointed outhereinafter or will be apparent to one skilled in the art upon readingthese specifications, we accomplish bythat treatment and have not comeinto wide commercial usage due to their resulting in a material ofextremely poor flatness. It was hitherto supposed that the only way ofincreasing the flatness characteristics of materials poor in theserespects was by giving them a carefully controlled box anneal. Evenhere, however, the flatness characteristics were not fully satisfactory,and furthermore it is not always desirable, either from the standpointof resultant magnetic and physical characteristics or from thestandpoint of economy, to conclude certain manufacturing processes witha box anneal.

We have discovered that under certain circumstances an open annealingtreatment may be caused to effect a marked improvement in the flatnesscharacteristics without involving undue expense; that our treatment,where desired, may be practiced upon otherwise finished materialswithout impairing their previously developed magnetic characteristics;and that where an open annealing treatment is desired as part of theprocessing treatment of the steel for the sake of other characteristics(such as ductility, magnetic characteristics and the like), the openannealing for the production of these desired results can be so carriedon as to respond to the invention herein set forth without diminishingits efiectiveness for the said results.

We have found that if a heat treatment in an open annealing furnace iscarried on, on the materials while they are subject to a controlledtension, greatly enhanced flatness will be the result without detrimentto the materials in any way. While it is possible, it would be verydifficult and costly to tension single hot rolled sheets at an elevatedtemperature. If the sheets are carefully welded together, preferably endto end to form a long band, the means of applying tensiOn during acontinuous annealing becomes relatively simple.

In forming a continuous supply from individual sheets or strips weprefer accurately to butt weld the sheets or strips together by theprocedures and using the apparatus of the, following patents: U. S.Letters Patents Nos. 2,172,080, 2,172,081, 2,219,493, 2,175,615,2,175,616, 2,196,941, 2,282,611 and 2,254,314.

By these procedures, we secure very long lengths of silicon steelcharacterized by welds at intervals, the welds however beingsubstantially no thicker than the welded sheets, and being so perfectlyformed that the steel in the endless strip is useful for'all of thepurposes to which the sheet material itself can be put, quite irre- 3spective of the position of the welds. Welded strip formed in this way,however, has not hitherto been characterized by great flatness.

Moreover, it would normally be supposed that the strains introduced intothe strip by .thewelding would be productive of a marked decrease inflatness if the welded strip were subjected to an open annealing, Wehave found however, that by subjecting the welded strip to tensionduring the heat treatment we can improve the flatness to the point whereit exceeds that produced by a good box anneal.

The tension required is not great. Indeed with certain kinds ofmaterials, where a fairly long furnace is used, the desired degree offlatness may be produced by the weight of the strip only, the stripbeing supported both as it enters and as it leaves the furnace. Weprefer however, so to operate as to apply a positive tension to thestripes it passes through the furnace. This is readily accomplished byproviding feeding devices for the strip both at the entrance and at theexit ends of the furnace and driving the feeding devices in asynchronized way so that the exit feeding device operates very slightlyfaster than the entrance feeding device, Pinch rolls may be used; andthey may be driven at the required related speeds by a connecting chainor shaft drive. In a type of open annealing furnace currently being usedby us, continuous supports have been eliminated, and the welded strip issupported upon driven rollers spaced about two feet apart. By means ofsynchronized pinch rolls, we apply to the strip sufficient tension tokeep it from sagging'to any great extent between the interspacedsupports, and such tension has been found" suincient to produceexcellent flatness.

Such tension is obtained by operating our exit rolls at a peripheralspeed, usually 0.1 to 0.5% faster than the-entrance rolls.

As indicated, the amount of tension required is not great. The strengthof the material at the elevated temperature is low; for example, 4.5%silicon steel has a tensile strength of about 15,000 pounds per squareinch at 1200 F., 3,000 pounds at 1500 F. and 1,500 pounds at 1800 F.While more tension is required than merelythat sufficient to move thestrip when supported substantially throughout its length on low frictionsupporting means, yet for some requirements the weight of the materialthroughout a, fairly long span is often sufiicient. One reason webelieve that the amount of tension required is relatively low, is thatthis type of material at elevated temperatures has no definite yieldpoint but begins to creep at relatively low stresses, Consequently thetensile strengths cited are only approximate since the materialpermanently elongates somewhat even with very low tension if enough timei given.

Thus the amount of tension required will vary with the composition andgage of the material, temperature to which the strip is heated, and thelength of time at which it is held at temperature. t will also varysomewhat with the degree of waviness of the material -to be flattened.For best results we have discovered that under whatever conditions theprocess is used, the tension must be sufiiciently great to permanentlyelongate the material slightly. For example, we commonly obtain apermanent elongation of 0.15 to 0.3% on one grade of material when givenour treat ment.

While not wishing to be limited by a'ny-theory, we believe that thezlack o-f flatness lof light :ga'ge,

hot rolled sheets is due to certain areas or surfaces being shorter thanthey should be, and that the flattening action is due to the effect ofthe tension in elongating the short areas until they are all the lengthrequired in a .flat sheet and the stresses set up by the lack ofuniformity are relieved.

An exact rule for the determination of the correct amount of tension tobe employed to flatten all wavy materials using all types of annealingapparatus cannot be arrived at on account of the interplay of the manyvariables, but with any given material, at any given temperature, itwill be within the skill of the worker in the art to select and apply anaverage tension which is sufficient to give the flatness desired withoutbeing great enough to produce longitudinal wrinkles or corrugations inthe strip, or pulling it in two,

The temperature of the heat treatment to produce flatness is not at allcritical; for example we have used temperatures as low as l200 In mostcases the-temperature will be governed by a consideration of the otherresults expected to be produced, or desired, from the open annealing.Where the primary purpose of the treatment is to produce flatness, it iswell to choose a temperature which is below the graphite solubilitytemperature, so that the treatment ,does'not have the result of bringingthe graphitic carbon into solution. Under these circumstances, themagnetic properties previously developed in the material will not beimpaired, and since there has not been a resolution of the graphiticcarbon, the treatment will not be productive of magnetic aging.

For example, when operating upon a silicon steel containing say from 4.5to 5.0% silicon, which steel has already been given a high temperaturebox anneal for developing the required core loss and permeability, Wecarefully weld individual sheets together and pass them under tensionthrough an open annealing furnace kept at 1490 to 1500 F. orthereabouts, holding the steel in the hot zone around one and one-halfminutes. Such a treatment greatly improves flatness while not harmingthe other properties of the material and in some cases improving them.

In those cases where the carbon content is quite low, such as when nocarbide or graphite particles are visible on microscopic examination,much higher temperatures can be satisfactorily used. 'Or if flatness isthe only consideration then the maximum permissible temperature ispretty muchdetermined by mechanical factors.

On the other hand, in processes for producing magnetic steels, where anopen anneal is desirable for other reasons as one of the steps or as thefinal step of a particular routing, temperatures appropriate to the saidstep may be used and our flatness attained by the use of proper amountsof tension. For example, when operating with a silicon steel of asilicon content below say 4.0%, which silicon steel has beendecarburized by box annealing it at an intermediate gauge in thepresence of the hot mill scale (in accordance With the teachings ofPatent 2,242,334 to Carpenter) and has been reduced to gauge,wecarefully weld thesheets into a strip and givethe strip a final openannealing .under tension at around 1500 to '2100 F. This treatment notonly produces the desired flatness but is an integral step in theproduction-of the desired magnetic properties.

The tension bears a certain relation to gauge and temperature; ?but withmaterial of any given gauge, having selected a temperature for treatmentas taught above, the skilled Worker can then select a tension to givehim the desired result. As to the duration of the heat treatment, ourinvestigations have shown that no great holding time is necessary if thematerial is brought to a softening temperature during the application ofthe tension. The length of time the material is in the furnace andsubjected to the temperature thereof will vary of cours rimarily withthe length of the furnace and the speed of travel of the strip. Thelength of time it takes to bring material up to temperature is of courseaffected by the gauge. With silicon steel of ordinary transformer gaugesthe material will usually reach temperature within one-half to oneminute. We have found it preferable not to cool the strip too abruptlyas by bringing it immediately out into the air from the hot zone.-- Weprefer to attach to our furnaces a cooling hood whereby the temperaturedrop may be made somewhat more gradually. The atmosphere within thefurnace does notaffect the flatness, hence forms no limitation on ourprocess. The atmosphere may be chosen in accordance with its effect uponother qualities such as brightness, core loss, ductility and the like.We prefer to use a neutral or reducing atmosphere to substantiallyprevent any scaling of the strip.

It will be seen that our process is much more effective than thoseprocesses employing, for example, an open anneal and a light cold rollinprior to a box anneal, because the products of those processes havehitherto been characterized by lack of flatness. Thus where the flatnessof the product resulting from a box anneal to develop magneticproperties is not satisfactory for a given use, the sheets may beweldedtogether and passed under tension through an open annealingfurnace merely to increase their flatness. Again where an open annealafter a box anneal is desired for any other purpose, the open annealingmay be carried on at the appropriate temperatures for these results, butunder tension in accordance with the present teachings. Yet again, aswhere the material has, for example, been decarburized at anintermediate gauge and then hot rolled to final gauge so that a finalpen anneal is all that is required to develop its magnetic properties,the said final open anneal may be made productive of flatness inaccordance with the present teachings.

The examples which we have given are not limiting since, as we haveexplained, our treatment for the production of flatness can either be anadded treatment for the specific purpose, or can take the place of anannealing already a part of the desired process for the production ofthe steel. Our treatment will thus be most generally applied to materialwhich has been reduced to final gauge.

Modifications may be made in our invention without departing from thespirit of it.

Having thus described our invention what we claim as new and desire tosecure by Letters Patent is:

1. A process for increasing the flatness of finished silicon steel instrip form, made by joining together pieces of lesser length withtransverse butt welds, and which is characterized by lack of flatness,which comprises bringing the sheet material to a temperature at which itbecomes soft enough for the relief of strains but below the graphitesolution temperature, and while at said temperature subjecting thematerial to tension sufilcient to flatten it, and produce a slightoverall elongation in the material, without producing in the'materialany detectable change in the crystal orientation.

2. A process for increasing the flatness offinished silicon steel instrip form, made by joining together pieces of lesser length withtransverse butt welds, and which, in finished form, is characterized bylack of flatness, which comprises bringing the material to a temperatureat which it is soft enough for the relief of strains but below thegraphite solution temperature, and subjecting the material to tensionsufficient to flatten it and produce therein a slight over-allelongation while at said temperature and while cooling said material tonormal temperatures at a rate slower than open cooling in air at roomtemperature, without producing in the material any detectable change inthe crystal orientation.

3. The step of increasing the flatness of finished, hot rolled siliconsteel sheets which comprises joining the sheets end to end to form aband, and subjecting the band to a softening temperature of the order of1200 F., and higher, but insufficient to cause graphite solution, andconcurrently subjecting the band to tension sufficient to increaseflatness and produce a slight over-all elongation in the band, withoutproducing in the material any detectable change in the crystalorientation.

4. The step of increasing the flatness of wavy, hot rolled silicon sheetsteel which comprises joining sheets end to end to form a band andsubjecting the steel to a softening temperature while tensioning theband sufiiciently to increase flatness by passing the band through acontinuous open annealing furnace including a cooling hood andpositively feeding the hand both into and out of the furnace at speedsinterrelated to produce tension in the strip and a slight over-allelongation therein, the value of the tension employed being not greaterthan that required to eliminate substantial sag in the material in aspan substantially equivalent to the furnace length.

5. The step of increasing the flatness of wa y silicon sheet steel whichcomprises joining sheets end to end by butt welding to form a band, andpassing the band so formed through an open annealing furnace whilecontrolling the entrance speed of the band into the furnace and its exitspeed therefrom to produce a tension and elongation in the band, whilesubjecting the band to a softening temperature, the value of the tensionemployed being not greater than that required to eliminate substantialsag in the material in a span substantially equivalent to the furnacelength,

6. A process as set forth in claim 5 in which the temperature to whichthe silicon steel is heated is above 1200 F. but less than the graphitesolubility temperature of the silicon steel.

7. A process as set forth in claim 5 in which the silicon steel prior tothe treatment for flatness is box annealed, and in which the temperatureto which the silicon steel is heated during the treatment for flatnessis above 1200 F. but somewhat less than the graphite solubilitytemperature of the silicon steel.

8. A process for flattening silicon steel which comprises formingsilicon steel sheets characterized by transverse waviness into a striplike supply of indefinite length and sheet gauge by butt welding, andpassing the said supply through an open annealing furnace at atemperature between 1200 and 1500 ,F., providing interspaced :feedingdevices acting on the supply as it passes through the furnace, andoperating said feeding devices at difierential speeds so related as toprevent cumulative sagging of the supply if it were unsupportedin saidfurnace, whereby to produce a tension substantially limited to thetension which would be exerted on the material by its .own weight ifunsupported in said furnace.

9. A process for flattening silicon steel which 10 comprises formingsilicon steel sheets characterized by transverse waviness into a striplike supply .of indefinite length and sheet gauge by butt welding, andpassing the said supply through an open annealing furnace at atemperature of 15 around 1200 to 1500 F. while subjecting the supply totension limited to an amountsubstantially suilicient to compensate forsagging of thematerial under its own weight at the said temperatures ifunsupported in a span of furnace length, the v20 10. In a process ofproducing silicon steel inpcludingthe step of simultaneously producingthe supply of indefinite length, passing said supply through an openannealing furnace and subjecting it therein to a temperature of betweensub-- stantially 1500 and substantially 2100 F. while .tensioning thesupply to produce flatness therein by positively controlling itsentrance'and exit speeds to substantially those differential speedswhich would be required to prevent cumulative sagging of the said supplyunder its own weight at the said temperatures if unsupported in the saidfurnace.

CARL E. GIFFORD. VICTOR W. CARPENTER. LOWELL-L. COOK.

